When manufacturing a steel plate and the like, a heating of the steel plate is conducted in various places such as, for instance, an annealing furnace and an alloying furnace for plating, and when a coated steel plate is dried. As a heating method of the steel plate, a gas heating, a trans-induction heating and the like, for instance, can be cited. For example, the gas heating is often employed in an annealing furnace, and the trans-induction heating, which can be used for a heating before plating, is mainly employed in an alloying furnace for plating, when drying a coated steel plate, and the like.
Meanwhile, an induction heating method can be largely divided into a solenoid system (axial direction magnetic flux heating) and a transverse system (transverse magnetic flux heating) or the like. In the solenoid system, a steel plate is heated by applying, to the plate, a magnetic flux along a longitudinal direction of the steel plate. In the transverse system, a steel plate is heated by applying, to the plate, a magnetic flux along a direction penetrating the steel plate. An induction heating method of transverse system is normally employed to heat a nonmagnetic material, and as a heating of a steel plate, an induction heating method of solenoid system is mainly employed. As the induction heating method of solenoid system, methods as disclosed in Patent Documents 1 and 2, for instance, have been conventionally known.
In the induction heating method of Patent Document 1, series variable capacitors are provided for respective heating coils used for the induction heating to equalize an amount of current flowing through the respective heating coils. However, in such a method, if a high-frequency alternating voltage of 50 kHz or the like, for example, is applied to the heating coils, capacitive reactance values in the series variable capacitors are reduced, so that series variable capacitors with larger capacities are required for appropriately controlling the amount of current. Meanwhile, when a steel plate is heated to a high temperature range in the vicinity of the Curie point, for instance, or when a heating rate is increased, there is a need to flow a large current through heating coils, to increase a frequency of an applied voltage and the like, for example. However, in the induction heating method of Patent Document 1, it is not possible to apply a high-frequency voltage because of the above-described reason, and accordingly, the amount of current has to be increased. It is difficult to design an entire apparatus to enable to flow a large current, and thus it has been difficult to heat a steel plate to a high temperature range and the like, for instance.
Meanwhile, in the induction heating method of Patent Document 2, two or more of single-turn coils are disposed along a longitudinal direction of a steel plate, in which a magnetizing force of the heating coil at a last stage is set to be one to ten times of a magnetizing force of the heating coil at a first stage. According to the induction heating method of Patent Document 2, it is possible to heat the steel plate to a high temperature range in the vicinity of the Curie point, and to reduce a decrease in a rate of heating in the vicinity of the Curie point. Note that the decrease in the rate of heating is reduced in the induction heating method of Patent Document 2 because the decrease in the rate of heating at the time of heating the steel plate causes ambiguous recrystallization behavior, interface control and the like, for instance, which makes it difficult to realize an optimum quality. However, in the induction heating method of Patent Document 2, variable resistors are inserted between the respective heating coils and a power source, and values of the variable resistors are changed to control the magnetizing force of each of the heating coils. Therefore, according to the induction heating method of Patent Document 2, the Joule heat is generated in the variable resistors, which causes a large energy loss (heat generation loss). Accordingly, although this may be acceptable in a case where a small current is flown, since a large current of, for instance, 4500 A is flown when heating a steel plate, such energy loss becomes large, and a large current has to be additionally flown through the heating coils in accordance with the energy loss, and it is further desired to improve an energy efficiency. Further, also in the induction heating method of Patent Document 2, it is difficult to completely keep the rate of heating constant since a magnetomotive force of the heating coil can be adjusted only by the resistance value of the variable resistor and a frequency of current flown through the heating coil, and an induction heating method capable of further reducing the decrease in the rate of heating is also desired.
Further, also in another method such as a method of controlling a final heating temperature to control the rate of heating and a method of adjusting the rate of heating, a control of a final heating rate and a mean value of the rate of heating was only conducted. Meanwhile, in a conventional manufacturing process of an alloying hot-dip galvanized steel plate, a heating furnace for heating for alloying has a long total length of, for instance, about 5 to 10 m, and with the use the heating method of controlling the mean value as described above, it was difficult to keep the rate of heating until a plating bath temperature reaches a final heating temperature constant, not only in a high temperature range in the vicinity of the Curie point. It is important to keep the rate of heating constant to strictly control an alloy structure, and also from that reason, an induction heating method capable of keeping the rate of heating constant is desired.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-243137    Patent Document 2: Japanese Patent Application Laid-open No. 2005-206906    Patent Document 3: Japanese Patent Application Laid-open No. 2001-21270    Patent Document 4: Japanese Patent Application Laid-open No. H11-257850